1.Field of the Invention
The invention relates to actuators and in particular to actuators for droplet deposition apparatus.
2.Related Technology
Droplet deposition apparatus or inkjet print heads are capable of placing small droplets of fluid onto a substrate. The apparatus, which will be called an inkjet print head even though fluids other than ink may be ejected—force the fluid from nozzles which communicate with an ejection chamber. Actuators corresponding with the ejection chamber apply the force that ejects the fluid. These actuators take a number of different forms but tend to fall within one of two categories. The first of which is mechanical, where an electrical pulse causes the actuator to deform, and includes such technology as electrostatic, thermal bend or piezoelectric for example. The second category is thermal or bubble actuators, where heat is applied to bring the fluid to its nucleation point. The resultant bubble pressurizes the ink in the chamber and forces some of it through the nozzle.
Piezoelectricity is a property of certain classes of crystalline materials including natural crystals of Quartz, Rochelle Salt and Tourmaline plus manufactured ceramics such as Barium Titanate and Lead Zirconate Titanates (PZT). Certain plastics such as PVDF can also express piezoelectric characteristics.
When mechanical pressure is applied to one of these materials, the crystalline structure produces a voltage proportional to the pressure. Conversely, when an electric field is applied, the structure changes shape producing dimensional changes in the material.
The piezoelectric effect for a given item depends on the type of piezoelectric material and the mechanical and electrical axes of operation. For certain types of piezoelectric material—notably PZT—these axes are set during “poling”, the process that induces piezoelectric properties in the ceramic and the orientation of the poling field determines their orientation.
After the poling process is complete, a voltage lower than the poling voltage changes the dimensions of the ceramic for as long as the voltage is applied.
A voltage with the same polarity as the poling voltage causes additional expansion along the poling axis and contraction perpendicular to the poling axis. A voltage with the opposite polarity has the opposite effect: contraction along the poling axis, and expansion perpendicular to the poling axis. In both cases, the piezoelectric element returns to its poled dimensions when the voltage is removed from the electrodes. When a voltage is applied in a direction orthogonal to the poling direction the piezoelectric element moves in thickness shear or face shear.
Generally two or more of these actions are present at the same time. In some cases one type of expansion is accompanied by another type of contraction which compensate each other resulting in no change of volume. For example, the expansion of length of a plate may be compensated by an equal contraction of width or thickness. In some materials, however, the compensating effects are not of equal magnitude and net volume change does occur. In all cases, the deformations are very small when amplification by mechanical resonance is not involved.
FIG. 1 describes the standard directions of piezoelectric material. The three orthogonal axis are termed 1, 2 and 3. The polar, or 3 axis, is always taken parallel to the direction of polarization within the ceramic. The indexes 4, 5 and 6 represent a shear movement around the 1, 2 and 3 axis respectively. The direction of polarization is established during the poling process by a strong electrical field applied between two electrodes. To link electrical and mechanical quantities double subscripts (e.g. dij) are introduced. The first subscript gives the direction of the excitation, the second describes the direction of the system response. For example, d33 applies when the electric field is along the polarization axis (direction 3) and the strain (deflection) is along the same axis. d31 applies if the electric field is in the same direction as before, but the strain is in the 1 axis (orthogonal to the polarization axis)
It has been proposed in the prior art to manufacture droplet deposition apparatus, or fluid pumps from piezoelectric material. One structure, described for example in U.S. Pat. No. 4,842,493 provides a pump channel formed by first and second piezoelectric parts arranged parallel to one another. The parts are polarised such that the polarisation direction lies parallel to a field generated by the electrodes. Upon application of the field the piezoelectric parts expand both in d31 and d33 modes and thereby affect the pressure of the ejection chamber. For example, d33 applies when the electric field is along the polarization axis (direction 3) and the strain (deflection) is along the same axis. d31 applies if the electric field is in the same direction as before, but the strain is in the 1 axis (orthogonal to the polarization axis)
A shared wall device operating in shear or d15 mode is described in U.S. Pat. No. 4,887,100. Two adjacent pressure chambers are separated by a single displaceable wall which can deflect towards or away from each of the chambers. When the wall deflects towards a first one of the adjacent chambers the pressure in this chamber is increased whilst the pressure in the other chamber is reduced. Similarly, when the wall deflects towards the second chamber the pressure in this chamber is increased with a corresponding reduction in the pressure in the first chamber. The pressure changes are primarily due to volume changes caused by the moving wall.
The provision of a shared wall allows for an increase in the chamber density and a reduction in the size of the print head for a given number of ejection chambers. However, as each wall acts on two chambers simultaneously it is not possible to fire droplets from each ejection chamber at the same time and hence this reduces the rate at which droplets can be ejected.